This invention relates to an optical information recording/reproducing system adapted to perform the recording/reproduction of information by radiating optical beams.
There have been known optical recording media each of which is adapted to perform the recording/reproduction of information by using a laser as a light source and radiating a light beam of 1 .mu.m or so in diameter to the surface of the medium. These optical recording media permit higher recording densities compared with magnetic recording media and the like and are attracting attention as large-capacity memories. As optical recording media, Te-C, TeO.sub.x (x=1.1-1.2), Te-Se-Pb, Te-TiAgSe, etc. have already been developed and found utility in such fields as disk memories for documents. As the first problem common to all the optical recording media which have been developed to date, it may be mentioned that they depend on a recording/reproducing method in which upon recording, a high-energy laser beam is radiated to locally heat a medium so as to form a hole or to induce a phase transformation in its recording layer and upon reproduction, a low-energy laser beam is radiated to detect a difference in reflectivity between the site where the hole has been formed or the phase transformation has been induced (the recorded site) and another site where no hole has been formed or no phase transformation has been induced (a non-recorded site) [reference may be made to many publications, led by the Handout of 116th Study Meeting (May 26, 1983), 131th Thin Film Committee, the Japan Society for the Promotion of Science]. In the above recording/reproducing method, it is indispensable to control the energy of the laser beam, which is used upon reproduction, lower than the threshold for recording. Since the S/N ratio of a reproducing signal is a value proportional to the energy of the reproducing laser beam, there is an upper limitation to the value of the S/N ratio as far as the above recording/reproducing method is relied upon. Due to such an upper limitation, the S/N ratios of media which have been proposed to date are as low as about 55 dB or so. It is therefore extremely difficult to apply such prior art media to memory fields requiring high S/N ratios, such as analog image file memories and high-quality image memories unless their S/N ratios are improved. As the second problem common to the optical recording media which have been developed so far, it may be mentioned that each medium contains a single recording-bit producing layer and the storage capacity of the medium is governed by the spot diameters of the recording and reproducing beams on the surface of the medium, the width and pitch of optical head guiding grooves and the area of the medium. For example, when a semiconductor laser beam having a wavelength of 830 nm is used and a substrate equipped with optical head guiding grooves of 1 .mu.m in width and 2 .mu.m in pitch is employed, the storage capacity of a disk having a diameter of 30 cm is limited to about 2 gigabytes or so. It is therefore extremely difficult to use such a disk in a memory field requiring high storage capacity such as high accuracy and fineness image memories and animation memories, unless its storage capacity is improved.
As mentioned above, the conventional optical recording media are accompanied with such problems as insufficient S/N ratios and storage capacity upon expanding their application fields as memories.